NEW YORK - At midmorning, the line at Liquiteria is almost out the door.

Customers at the bright, cheerful juice bar can't seem to get enough of owner Doug Green's menu of smoothies and fresh-squeezed juices. Some of the drinks are billed as energy builders; others promise to burn fat, boost the immune system or detoxify the lymph system.

On the counter, a tabloid photo shows Natalie Portman clutching a bottle of Liquiteria's signature cold-pressed juices (meaning the liquid is extracted by chopping and pressing, rather than spinning in a centrifuge). Beside the picture is a framed thank-you note from another actress, Rachel Weisz, "for all the juice."

But New York's hard-core juicers don't need to trek to this trendy East Village neighbourhood where Liquiteria has been serving a loyal clientele since 1996. There are dozens of juice bars all over the city, including 18 branches of the California-based Jamba Juice.

Once the drink of hard-core health nuts, fresh-squeezed vegetable juice - along with its far more popular sibling, fresh-squeezed fruit juice - has come of age.

Today there are more than 6,400 outlets across the United States that sell fresh juice and smoothies, ringing up US$3.4 billion in annual sales, according to industry consulting group Juice Gallery Multimedia. It's a far cry from the early days, when juice bars were often drab affairs tucked in the back of health food stores, emitting the grinding, horror-movie sounds of fibrous beets and carrots meeting industrial-strength blades.

Dan Titus, the head of Juice Gallery Multimedia, traces the evolution of specialty juice bars to the health and fitness movement that began to take off in the 1960s as surfers and hippies experimented with natural foods, vegetarianism and macrobiotic diets. Juicing exploded in popularity in the late 1980s and early '90s with the introduction of the Juiceman machine in a hard-sell infomercial (now satirized on the web) on late-night TV. Fitness legend Jack LaLanne jumped in, introducing a model under his name.

Richard Radulovich, general manager of a Lodi, Calif.-based company that produces the Champion juicer, says there were only a handful of manufacturers when the Juiceman was introduced. Soon the number shot to over 50 brands, including some from big companies like Singer, Samsung and Panasonic. Only 15 or 16 models remain now, with business slumping because of the recession. But "juicing is still popular," he said. "There's nothing healthier than eating raw vegetables."

Neil Mitchell, a 49-year-old personal trainer at the 92nd Street Y in Manhattan, started making his own vegetable juice about 15 years ago after seeing a Juiceman infomercial. His current blend includes carrots, beets, kale, broccoli, parsley, celery and ginger, and he believes it gives him more energy.

"It's a hard thing to describe but you just feel healthier," he said. "When I was in my late 20s I used to wake up with stiff joints. I'd have to take a hot shower when I got up. That went away in about five weeks. It absolutely disappeared."

Whether drinking fresh juice every day delivers all the health benefits that advocates claim is a matter of debate. Nutrition experts say North Americans in general don't eat enough fruits and vegetables, so they applaud any diet that increases servings of those two vital food groups.

But they urge people to follow commonsense guidelines when consuming unpasteurized juice: Wash produce before juicing it, and use such drinks as part of a balanced diet. As for the more extravagant claims made by some proponents that fresh-squeezed vegetable juice detoxifies organs and cleanses the digestive tract, medical professionals are skeptical.

"I honestly don't understand the concept of intestinal cleansing. It's not like you'd find old tin cans or spare tires in the colon," said Dr. Edward Saltzman, a research scientist at the Jean Meyer USDA Human Nutrition Research Center at Tufts University. "Anything that results in increased motility or movement in the intestines, such as intake of fibre and fluid, would result in the evacuation of bowel contents."

One thing is for sure: Nutrition is a complex topic that doesn't lend itself to sweeping claims. People who believe in juicing say it's easier for the body to absorb the vitamins and minerals in vegetables when the nutrient-rich juice is separated from the food itself, but Saltzman hasn't seen much evidence of that. He notes that some nutrients may remain in the pulp left behind in the juicer (although some juicing proponents recommend using the leftover pulp in soups and baked goods, or baby food).

Saltzman also notes that some nutrients, such as vitamins A, D, E and K, are fat soluble, meaning they are better absorbed when consumed with a small amount of fat. That means if you're juicing a vitamin K-rich green leafy vegetable like Swiss chard or kale, you would absorb more of its nutrients if you also ate a very small amount of fat or another food prepared with a little oil.

Eric Friedman, director of the Office of Consumer Protection for Montgomery County, Md., got his juicer about 10 years ago when his oldest child, Ali, was a toddler. He says both his kids loved the way the machine sucked in the vegetables and used to have fun doing it with him. He would juice a blend of carrots, beets, beet greens, parsley, dandelions and bok choy for himself, then slip some of it into their kid-friendly apple juice. He can't prove it made him or his kids healthier, but he believes it did.

Friedman recently established a new juicing ritual, this time with his 84-year-old mother. Every day or two, he goes to her house to "do shots" of wheatgrass.

For true believers, wheatgrass is the Holy Grail of nutrition. One website claims it washes drug deposits from the body, neutralizes toxins, purifies the liver and prevents tooth decay. Similar in appearance to a well-manicured lawn, it's sold by the flat in health food stores and harvested with a scissors.

Friedman's mother, Elsa, a retired dentist with a longtime interest in holistic medicine, keeps a tray in her refrigerator. "She'll call me up and say, 'I got the grass, I got the good stuff,' and I'll come over," Friedman joked. "It's a very nice excuse to visit my mother."

Saltzman, at Tufts, said a small number of very preliminary studies suggest that wheatgrass may have a role in reducing the symptoms associated with autoimmune or inflammatory disorders, or possibly even the disease process itself. "But this area of research needs to be expanded before I would recommend wheatgrass to supplement current treatment for these disorders, and I would certainly not recommend replacing standard therapies with wheatgrass based on current evidence," he said.

A word of caution if you decide to juice: Washing and chopping the vegetables and cleaning the machine can be time consuming.

"From the time I took everything out of the refrigerator, washed it all, cut it up, ran it through the machine and cleaned the machine, it would take at least an hour," Friedman said.

Mitchell, who gets up very early to be at the gym when it opens, has streamlined the process by making about 700 millilitres of juice at night. He drinks half of it right away and saves the rest for the morning.

"On a good week, I'll cut and clean five days' worth of stuff and put it in separate bags" in the refrigerator, he said. He has also become adept at taking apart his top-of-the-line juicer, a so-called masticating model which squeezes rather than spins the vegetables.